In electronic reproduction technology, a method and circuit arrangement are provided for selective correction of hues and colors in a production of color separations for polychromatic mixed printing or multicolor printing.
In the production of color separations "yellow", "magenta", "cyan", and "black" for polychromatic mixed printing by use of a color scanner, color signals which are converted into color separation signals by means of a basic color correction are acquired by opto-electronic scanning of a color original. The color separation signals which are a measure for the intensity of the color application of the inks yellow, magenta, cyan, and black in the printing process are supplied to recording elements, for example lamps, whose brightnesses are modulated as a function of the color separation signals. The write lamps expose films as recording media point-by-point and line-by-line, these films representing the color separations "yellow", "magenta", "cyan" and "black" for the polychromatic mixed printing after they have been developed.
By use of the basic color correction among other things, color errors are eliminated which result because of different spectral properties of original colors and inks and, under given conditions, the desired edited chromatic information of the reproduction is altered in comparison to the original. In addition to the basic color correction, a selective color correction is often executed, and very specific colors or hues are corrected therewith.
In the selective color correction, the problem exists of generating selective correction signals which are only effective and influence the basic corrected color separation signals when the specific hues or colors to be corrected have been recognized in the color original.
When the color original to be reproduced has a color progression, i.e., differing color saturation and/or luminance within a color or a hue, selective correction signals whose intensity also depends on the color progression are required in order to achieve progressive gradually changing correction intensities.
An apparatus for selective color correction given a color scanner for the production of color separations for polychromatic mixed printing is already known from German Letters Pat. No. 26 28 053, corresponding to U.S. Pat. No. 4,194,839, incorporated herein by reference. A color recognition circuit is connected to receive the measured color value signals acquired in the scanning of the original. Within the color recognition circuit a recognition region for a color to be selectively corrected (correction color) in the color original is defined within the color space. The recognition circuit then selects the selected correction color from the other colors of the color original during the scanning of the original and always generates a selective correction signal when the selected correction color is recognized in the color original. The known apparatus has the disadvantage that it is essentially only prismatic color recognition spaces that can be defined. The color recognition spaces can therefore not be optimally matched in terms of shape and size to the color regions prescribed by the color progressions of the color original, so that the color recognition signal does not supply an unequivocal statement regarding the color saturation and/or luminance of the scanned colors. The desired progressive selective correction signals can thus likewise not be derived from the recognition signal.
A further disadvantage of the known apparatus for selective color correction is that the color recognition circuit is not specifically designed for the definition or for the recognition of hues, so that no optimum selective correction signals for hues can be generated.